The present invention refers to a method of quenching metals, in particular alloy steels in a boiling water-based solution of sodium tetraborate, with the sodium tetraborate content in water amounting to between 20 and 50% by weight.
German patent no. DE 29 43 065 C2 discloses a method of quenching steels and metal alloys, with a water-based solution containing 22 to 50% by weight of sodium tetraborate and with the quenching process taking place at boiling temperature of the solution. The objective of this known method is the elimination of conventional quenching media (oil) because their use fails to address environmental standards during the quenching process and during a later disposal. Further, this known method is concerned with the control of great heat energies, e.g. in a wire rolling mill in which wire coils are quenched in rapid sequence, without necessitating a cumbersome quench bath cooling and experiencing smoke formation of the oil quenching. A drawback of this known method is the difficulty to remove solids floating in the boiling quenching medium and having a considerably higher density than water because the boiling quench bath may only be pumped at overpressure and at prevention of cooling and precipitation of sodium tetraborate crystals. Thus, the overall process and operation becomes significantly more complicated, and the maintenance costs are considerably increased.
It is generally known in connection with the quenching of metals or in the hardening technology, that it is necessary to keep the quenching medium clean. Since on the one hand solids inevitably are carried with the material being quenched into the bath and accumulate in the quench bath, and on the other hand volatile components of the quenching medium escape from the bath or components of the composition adhere to the material being quenched and exit the bath, the operation is generally carried out in such a manner that either the content of the quench bath is pumped out and disposed and then replaced by fresh quenching medium, or the losses are constantly replaced with fresh quenching medium to counter an aging of the quench bath. This procedure is however complicated and cumbersome.